Filtration is a commonly utilized technology to separate liquids and solids in a number of fields including mineral processing, coal slurry dewatering, as well as in the chemical, food, paper-making and environment protection industries. Liquid filters are of two classes: cake filters and clarifying filters. The present invention relates to filters from the former class that are typically utilized to separate slurries carrying relatively large amounts of solids, typically at least 1%, to be recovered. These solids build up on the filter medium as a visible, removal cake which normally is discharged as a moist mass. There are three types of cake filters. Depending upon the driving force producing the separation of the liquid/solids, there are centrifugal filters, vacuum filters or pressure/hyperbaric filters. The present invention relates to this latter type also classified as batch operating or intermittent filters.
During hyperbaric filtering, very fine particles reduce the size of the capillaries in the filter cake and often close down and partially block the pores in the filtering medium. This serves to increase the threshold pressure which the dewatering or filtering equipment has to exceed to achieve the necessary fluid flow to provide the desired final cake moisture.
Up until recently, hyperbaric filtration theory has focused solely upon the pressure as the driving force of liquid/solid separation. Specifically, an area of high pressure is maintained on one side of the filter cake held on the filtering medium and an area of low pressure is maintained on the other side of the filtering medium. The greater the pressure differential, the greater the flow of fluid and, hence, the separation efficiency.
It has now been discovered that by cyclically deforming the filter cake while under compression, drying of the filter cake is improved. Such a procedure is disclosed in, for example, U.S. Pat. No. 4,826,607 to Pearce. In Pearce a vacuum is applied to a drainage pipe beneath the filter cake held on the filtering medium. This serves to draw moisture from the filter cake. Simultaneously, an oscillating flow of fluid is directed into a separate chamber against an impermeable, flexible membrane that engages the permeable flexible filtering medium. The oscillating flow of fluid causes the membrane and the filtering medium as well as the filter cake held thereon to bend. This bending serves to form capillaries in the filter cake and maintain the pores in the filtering medium open so as to allow improved fluid flow through the filter and, therefore, moisture removal.
In ultra fine particle-liquid suspensions, it has also been found difficult to reduce the moisture content below a certain level by only mechanical means. This is because of the large specific surface area of such suspensions and the electrical charge carried by many of the particles. In order to improve the efficiency of separation of such suspensions it is known to apply a voltage across the filter cake. Such a procedure is shown in, for example, U.S. Pat. No. 4,003,819 to Kunkle et al. Advantageously, the voltage causes the liquid entrained in the filter cake to migrate electroosmotically with the fluid being drawn from the filter cake through the filtering medium.
While the prior art approaches such as disclosed in Pearse and the Kunkle et al patents are effective in increasing the efficiency and effectiveness of liquid/solid separations through filtration, further improvements in overall filtering efficiency may still be achieved.